Sum of Opponents' Scores
This number is the sum of the score (wins) of all opponents that this player played against in that tournament. It doesn't matter if the game was a win or a loss.
This number is often used as tie breaker in McMahon or other tournaments. The reasoning here is that if you played tougher opponents, your SOS will be higher, so you deserve a higher position that people who played easier opponents.
Example. You played in a three round tournament. You won against Steve, lost against Tracy and won again against Rob.
Steve won 2 of his games, Tracy won all 3 and Rob only won 1.
So after three rounds: